In the description that follows, the term paper rolls and, its equivalent term “logs” refer to rolls of web material (such as paper) wound up around a support core (e.g. a cardboard tubular core) to be cut in order to form rolls of smaller dimensions, that is, of commercial size.
The current production of small rolls of paper is known to include cutting transversely the logs produced by a rewinding machine and with the use of one or more cutting-off machines.
A cutting-off machine for logs is described in details in the document IT 1247330.
In practice, a cutting-off machine of this type includes:                a plurality of channels or guides into which the logs are unloaded, and in correspondence of which movable devices, so-called “porters”, are mounted and operated for moving longitudinally the logs by pushing them from the back;        a cutting station, comprising one or more circular blades rotating about axes parallel to those of the logs and fixed onto a corresponding movable support; and        means for discarding the waste, that is, the end trims produced in the cutting station.        
During the normal operation, a log to be cut is disposed by a relevant porter unit in a preset position onto the respective guide, the same log is retained in such position, and one of the blades is operated, that is, is driven in a plane orthogonal to the log's axis, so as to form a shorter roll of preset size. The length of the small roll depends actually on the advancement of the porter unit during the time elapsing between two consecutive actuations of the blades.
The current production processes imply some degree of inaccuracy in the formation of the logs. In fact, the cores are likely either to project from the ends of the wound-up material or to result thereinside. Moreover, the end bases of the logs may result oblique with respect to the longitudinal axes thereof and their consistence may lack in uniformity.
In conclusion, the length of the logs is never the same, both owing to the process irregularities and to the deformations induced by the pressure exerted by the porter units onto poor-consistence bases of the logs, which deformations bring about errors in the advancement of the porter units and are thus the cause of inaccuracy on the lengths of the small rolls formed from the logs. Such inaccuracies result crucial when the cuts must be made at preset distances from printed pictures or other imprint figures formed on the paper of the logs.
Besides, the first and the last cuts, required for trimming the logs in correspondence of their two ends, generate waste or trims which must be detached from the small rolls before putting the latter on the market. Provision is made therefore for using means intended to separate the trims, which means are never totally efficient and their intervention is likely to prejudice the quality of the small rolls.
A further drawback related to this log-trimming technique lies in the poor quality of the trimming cuts: in order to keep the production waste at a minimum, the length of the trims is minimal but, owing to the lack of homogeneity of the material close to the end bases, the corresponding cuts are hardly ever orthogonal to the axis of the logs, so that, most of the times, for each log there are produced two imperfect small rolls (one for each end of the log), also due to the high cutting speed and to the shape of the chamfer of the blades. The result is that, generally, the adopted solution is a trade-off which fulfills only in part the requirements for the trimming and cutting-off of the logs.